Nuclear cameras are used in the medical field for locating tumors or other biological abnormalities. A radioactive isotope combined in a suitable compound is either injected into the blood stream of the patient or is ingested orally. Certain compounds or elements are taken up preferentially by tumors or by specific organs of the patients body such as iodine in the thyroid gland. As the radioactive isotope disintegrates, gamma rays are ejected from the location in which the compounds or elements are concentrated. The gamma-ray or nuclear camera receives the rays, converts them to light and constructs scintigrams or scintiphotos which show the tumor or abnormalities in the organ.
More specifically, the nuclear camera includes a collimator which is a lead plate with pin holes which permits gamma rays to pass through it only when the rays are parallel to the pin holes in the lead. The gamma rays then strike a crystal, such as a sodium iodide crystal, which scintillates or converts the gamma rays to light and the light is transferred by light tubes to photomultiplier tubes which convert the light to electrical signals. The electrical signals are subsequently digitized and analyzed through appropriate algorithms to determine the intensity and direction of the gamma rays which are then projected onto a cathode ray tube. Different isotopes produce gamma rays having different energy levels. The thickness of the collimator is ideally sized to correlate with the energy levels of the gamma rays produced by the radioactive compound. If the collimator is too thick, an insufficient number of gamma rays will pass through the pin holes to permit satisfactory reconstruction of an accurate picture. If the collimator is too thin, angular gamma rays will have sufficient energy to pass through the pin holes distorting the scintiphoto.
Generally speaking, the nuclear camera art has developed to the point where at least three different collimators of varying thicknesses are supplied with the camera. For purposes of this invention, the collimators can be classified according to thickness as thin, medium or thick. The collimator is mounted or encapsulated within a frame which, in turn, is removably fastened to the lead shielded camera housing containing the crystal, light tubes and photomultipliers. The cameras, usually two, are mounted to a gantry which straddles the patient. The gantry is usually of the type that not only permits the cameras to move into or away from the patient and travel longitudinally along the length of the patient's body, but also to circumferentially rotate from horizontal to vertical positions about the patient.
Heretofore, when the collimator had to be changed, a cart with prongs resembling a tow motor truck was used. The camera was positioned in it's upper horizontal position and the prongs were adjusted to rest on the face side of the collimator. The fasteners holding the collimator frame to the camera housing were unscrewed and the cart, with the collimator lying face down on the prongs, moved away. The prongs then had to be lowered a significant distance and the collimator lifted off for placing into a storage shelf, whereupon a new one was then laid on the prongs and the process reversed.
It can be appreciated that this transfer arrangement is likely to damage the collimator. Collimators are constructed of leaves of lead foil assembled into a tight honeycomb pattern. Any accidental jarring could deform the lead foil ruining the pin holes in the area where the collimator might have been jarred. This jarring can occur when the collimator is "dropped" during transfer to the prongs such as when the collimator is removed or dropped onto the shelf. Also, because the collimator simply lies horizontally on a storage shelf, face side down, it is not that uncommon for the collimator to have been ruined by liquid spills such as can occur when the clinician places a coffee cup on the exposed back side of the collimator. Collimators are expensive items and are designed for the life of the camera. It is somewhat surprising that a relatively large replacement market for collimators exists. Obviously, the collimators are damaged during removal, storage and retrieval.
Apart from the damage, or probability of damage, done to the collimators by the transfer techniques heretofore employed, it is also to be appreciated that collimators weigh anywhere from 100 to 300 lbs. Since the collimator has to be physically moved, even to permit fastening to the camera housing, the chore can be difficult for the clinician especially when a high proportion of nuclear camera clinicians are women. In many instances, additional technicians have to be used to effect the transfer under the direction of the nuclear camera clinician.
Also, the transfer time is simply long. For instance, the cart's transfer prongs have to be cranked from a floor level to a relatively high horizontal camera level to simply position the prongs which then have to be returned to the low level, etc.
The cumulative effect of the problems discussed above is that many hospitals will use only the medium collimator and the doctors will simply have to interpret the scintiphoto as best they can. Alternatively, the hospitals will schedule the patients in accordance with the gamma ray energy levels of the radio isotope compounds injected into or ingested by the patients. However, emergency or critical cases disrupt the schedule. In general, because of the time, the difficulty and the damage probability attributed to changing the camera's collimator, the collimator is simply not changed as often as it should be.